Step 1:
Firstly, look at the various assessments you already conduct in your course. Are any of them suitable for conversion to video assessment? Assessments which are likely to be most suitable are often those which are otherwise done by practical demonstration, peer presentations, observation checklists, etc. Importantly however, you need to consider whether the conversion of your assessments to video is likely to improve the efficiency, accessibility or accuracy of the assessment process.

Step 2:
Design the marking key / competency checklist that will be used to review the video. This may be an existing document.

Step 3:
Search the net for examples of presentations or demonstrations which reflect what you want the students to do (rather than reinvent the wheel unnecessarily). The most obvious online resource is youtube.com, but there are others too such as google video, . This could be in the form of good examples, or examples of what you want the students not to do. If necessary, you may need to produce your own video for students to refer to.

Step 4:
Complete the marking key / competency checklist for the examples you intend to show students. This will be useful to show students what you expect from them.